Radio transmitting circuits



Dec. 2U, 194-9 H. Moss $491,754

RADIO TRANSMITTING CIRCUITS Filed Nov. 4, 1947 I/VVENT R fly fi Patented Dec. 20, 1949 RADIO TRANSMITTING CIRCUITS Hilary Moss, London, England, assignor to Cossor Limited, London, Englandya British company A plication November 4,1947 Serial No. 783,995..

p In Great Britain November 8, 1946 15 Claims.

This invention relates to radio frequency modulators and has among its objects that of obtaining amplitude-modulated radio frequency waves with very low distortion at modulation depths up to 100%. One way in which this object is attained is by the use of heavy negative feedback in the manner to be described.

The invention will be more particularly described with reference to the accompanying drawing in which Figures 1 and 2 show stages in the development of a circuit according to the invention and Figure 3 shows a circuit for obtain- .ing amplitude modulated radio frequency oscillations.

In th circuit shown in Figure 1 there are connected between the positive line and earth a potentiometer R1, R2 and a valve circuit of the cathode lead resistor type consisting of a valve V1 with a resistor R3 between its cathode and earth. The rid of V1 is directly connected to the point P in the potentiometer R1, R2. The valve V1 is a short grid-base triode, and the line voltage +V is very large in comparison with the grid bias of V1: then the cathode of V1 and the point P are at substantially the same potential. Hence, if a equals then as the voltage between P and ground has the value a.V, the current 1' through the valve V1 iS and is thus proportional to the line voltage V. Exact proportionality would obtain only if the grid bias of the valve were zero, but a close approximation to proportionality may be obtained by using high line or plate voltages, by making its large and by making a not less than about Now consider Figure 2, which is identical with Figure 1 except that the connection between the grid of V1 and P is broken at X-Y and a capacitor C is in shunt across R3. Suppose that a series of sharp square pulses is injected between X-Y in such a sense as to make the grid of V1 negative to point P, these pulses being of constant duration and recurrence rate in the radio spectrum. The valve V1 is thus periodically cut off. The cathode condenser C has such a value as to decouple R3 at the pulse repetition rate. Then the current through the valve V1 will be pulse modulated in phase with the grid pulses and by the reasoning discussed in connection with Figure 1, the peak current will beproportional to the line or plate 2 V voltage. This current will fall to zero on receipt of each negative-going pulse at the grid.

If therefore, the line or plate voltage of Figure 2 is modulated in amplitude, the same amplitude modulation will be impressed upon the current pulses in valve Vi. If a circuit resonant at the pulse recurrence frequency is included as anode load of V1, this resonant circuit receives a succession of current impulses modulated in amplitude strictly in accordance with the variation in the line or plate voltaga The oscillatory properties of the circuit then reconvert the current pulses into an amplitude modulated sinusoidal radio frequency oscillation.

Figure 3 shows a practical circuit incorporating these principles. The anode load of the valve Vi incorporates the tuned resonant circuit L1G; which resonates at the carrier frequency, and also the low frequency transformer K which injects the audio-modulation by variation of the instantaneous anode potential of the valve V1. The condenser 02 decouples the radio frequency output. The cathode bias resistor R3 is shunted by C1 and the value of C1 is so chosen as to prevent negative feedback at the radio frequency, but not at the modulating audio-frequency. The radio frequency input to V1 is injected across R4 by means of a tuned high frequency transformer. The rectifier T results in unidirectional current pulses through R4 and is so connected that the grid is driven negative with respect to P. If a large signal compared to the grid bias of V1 is developed across R4, then it can be considered that V1 is being pulsed negatively by a substantially square wave and the anode current will consist of similar square waves, the peak amplitude of which corresponds to the condition where the grid potential is the potential at P, i. e. rectifier T non-conducting. The tuned circuit L1C3 converts the square pulses into sinusoidal waves.

The circuit of Figure 3 has been tested with the following values of components:

R1=R2=30,000 ohms R3=50,000 ohms R4=25,000 ohms C1=0.01 pf.

The valve V1 42SPT.

Line voltage V=400 Peak R. F. volts across R4=40 Peak audio volts across secondary transformer Under these conditions a. modulated wave was obtained with less than 1% total harmonic distortion.

It is believed that this result cannot be achieved by conventional non-feedback modulators, even when they are adjusted to give optimum performance. It is also well-known that such adjustment is quite critical, whereas it is an outstanding advantage of the circuit now described that there is wide tolerance in network values, while owing to the use of heavy negative feedback, the results, are almost entirely independent of valve characteristics.

The recurrence frequency of the pulses applied to the grid of valve V1 need not necessarily be equal to the resonance frequency of the tuned circuit L103 arranged in the anode circuit of the valve for converting the pulses of anode current into sinusoidal waves, but may be some multiple or sub-multiple thereof provided the said tuned circuit will resonate at the desired carrier frequency in response to pulses of that recurrence frequency.

' I claim:

I 1. A radio-frequency modulator for modulating an oscillation of a desired radio frequency comprising a thermionic valve, circuit means for maintaining the anode current of the valve substantially proportional to its anode potential at the said frequency, a source of voltage pulses of a predetermined recurrence frequency, means for applying said pulses to the control grid of the valve, a source of modulation voltage, means for applying said modulation voltage to the anode of the valve, and a circuit which resonates at the desired frequency in response to an anode current pulsating at saidrecurrence frequency so as 'to' produce an amplitude-modulated substantially-sinusoidal oscillation of desired carrier frequency.

2. A radio-frequency modulator comprising a thermionic valve, a resistor having one terminal connected to the cathode of said valve, a potential divider connected between the anode of said valve and the other terminal of said resistor, a source of radio-frequency pulses applied between a point in the potential divider and the control grid of the valve, a source of modulating oscillations applied between a source of anode voltage and said anode, and a circuit which resonates at a desired carrier-frequency in response to pulses of the said radio-frequency for deriving from the amplitude-modulated pulsating anode current in amplitude-modulated substantially sinusoidal oscillation.

3. A radio-frequency modulator for modulatin an oscillation of a desired radio frequency comprising an electron discharge valve having an anode, a cathode and at least one control electrode, circuit means for maintaining the current between said anode and cathode substantially proportional to the potential of said anode at said radio frequency, means for applying voltage pulses to said control electrode and means for applying modulating voltage to said anode.

4. A radio-frequency modulator for modulating an oscillation of a desired radio frequency 5. A radio-frequency modulator for modulating an oscillation of a desired radio frequency comprising an electron discharge valve having an anode, a cathode and at least one control electrode, a resistor connected between said cathode and a point of fixed potential, 9. potential divider connected between said anode and said point of fixed potential, an impedance connected between a tapping intermediate the ends of said potential divider and said control electrode, means for applying voltage pulses across said impedance and means for applying modulating voltage to said anode.

6. A modulator according to claim 5, wherein said voltage pulses are substantially square wave.

7. A modulator according to claim 5, wherein said means for applying voltage pulses across said impedance comprise a source of substantially sinusoidal oscillations and a rectifier for producing said voltage pulses from said sinusoidal oscillations.

8. A radio-frequency modulator for modulating an oscillation of a desired radio frequency comprisin an electron discharge valve having an anode, a cathode and at least one control electrode, a resistor in parallel with a capacitor connected between said cathode and a point of fixed potential, a potential divider connected between said anode and said point of fixed potential, means for applying Voltage pulses between a point intermediate the ends of said potential divider and said control electrode and means for applying modulating voltage between said anode and said point of fixed potential.

9. A modulator according to claim 8, wherein said capacitor has an impedance small in comparison with the resistance of said resistor at the frequency of said pulses and greater than the said resistance at the highest frequency of said modulating voltage.

10. A modulator according to claim 8, wherein there is connected to said anode a circuit tunable to said resonance frequency.

11. A modulator according to claim 10, wherein said resonance frequency is equal to the recurrence frequency of said voltage pulses.

12. A modulator according to claim 10, wherein said resonance frequency is an integral multiple of the recurrence frequency of said voltage pulses.

13. A modulator according to claim 10, wherein the recurrence frequency of said voltage pulses is an integral multiple of said resonance frequency.

14. A modulator according to claim 8, wherein said means for applying voltage pulses comprise a transformer having a secondary winding tuned to the recurrence frequency of said voltage pulses.

15. A modulator according to claim 8, wherein said means for applying voltage pulses comprise a transformer having a secondary winding tuned to the recurrence frequency of said voltage pulses, a rectifier connected in said secondary winding, and a source of oscillations connected to the pri mary winding of said transformer.

HILARY MOSS.

Name Date Reeves Dec. 16, 1941 Number 

